Controlling the field of a generator has always been somewhat of a problem when the operating efficiency of the system is of key importance. In such cases, a high-efficiency circuit is imperative. To maintain a relatively constant voltage, the circuit should have voltage feedback to increase the current delivered to the field under loaded conditions, while current feedback could be included to protect the system from over-current conditions. The circuit should also be designed so that the generator can be controlled from full stop to full speed. Frequency compensation is also necessary.
Since a high-efficiency solid-state amplifier is needed to control the generator field, a study of amplifier classes is in order. The most common types of amplifiers are classes A, B, AB, C, and D. In the basic class-A transistor amplifier, a specific quiescent current Ib is always present in the base of the transistor. This base current results in a collector current (Ic) of Ib.times.B, where B is the current amplification factor of the device. With the amplifier not processing any signals, there will be some collector current flowing. However, this is wasted power since there is no demand for work to be done. Varying the base current varies the collector current, and causes the operating point to shift on the load line. As Ib decreases, the voltage across the device (Vce) increases.
According to Kirchhoff's Law, which states that the sum of the voltages around a loop must be zero, we can see that any voltage which is not dropped across the load controlled by the transistor must be dropped across the transistor. Therefore, not only is power being dissipated in the load, but power is also being dissipated as heat in the transistor. This mode of operation is very inefficient. In fact, the maximum operating efficiency for a class-A amplifier is 50%--this is unacceptable for generator field control. Examples of an elementary voltage regulators using transistors are U.S. Pat. No. 4,360,772 to Voss; and U.S. Pat. No. 3,076,922 to Seike.
The class-B amplifier, has a greater operating efficiency than a class A. In this case, the two transistors are biased at cutoff, eliminating the inefficiency of quiescent current. One half of an input signal causes one transistor to conduct and the other to be reverse-biased. The other half of the input signal causes the opposite to happen. Unfortunately, any voltage still not dropped across the load is dropped across the device. As in class-A operation, any power not dissipated in the load is wasted as heat. The maximum efficiency of this amplifier is 78.5% which is better but still an unacceptable design. U.S. Pat. No. 3,170,110 is an example.
From the brief review of class-A and B amplifiers, we can see that in any amplifier configuration, inefficiency is present when the difference in power supply voltage and load voltage is dropped across the device (assuming some current flow). Any amplifier configuration in which the device is operated in the linear region cannot be used since maximum efficiency is a mandatory requirement. Since classes AB and C also operate in this manner, they can be eliminated as possibilities for this design. Therefore, exit classes A, B, AB, C and enter class D--the switching amplifier.
The active device in a class-D amplifier is never operated in the linear region as it is either in cutoff or saturation. If the device is in saturation, the voltage across it is theoretically zero (maximum current) and no power is dissipated. When the device is reverse biased, there is maximum voltage but no current (theoretically) and no power is dissipated. Thus, the class-D configuration can be looked at as a simple on/off switch. By this simple representation we can see that all the power will be dissipated in the load, and we have achieved a maximum theoretical efficiency of 100%. The actual efficiency is less, of course, due to certain characteristics of the semiconductor switching device. We expect the voltage across a closed switch to be zero. In actuality there is some small voltage across the device in the saturated state (Vsat), just as there is some leakage current through the device in the cutoff state. Although this type of amplifier is far better than the others, another major problem remains--the selection of the solid-state device that will accomplish the actual power switching. What is needed is a solid-state device having low "on" resistance, resulting in low Vsat, and low leakage current. These two factors reduce the power dissipated in the solid-state device during the "on" and "off" cycles. Heretofore, bipolar switching devices have been used. Examples are presented in U.S. Pat. No. 3,984,755 to Lenhoff (SCR's); and U.S. Pat. No. 4,219,769 to Mac Farlane (TRIAC's). The main disadvantages of bipolars are their inherent "current hogging" characteristics. The current in a bipolar generates heat. The buildup of heat results in a lowering of internal resistance, which leads to an increase in current flow. This phenomenon, known as "thermal runaway," is characteristic of bipolars in general.
Since it may be necessary to parallel the solid-state switching devices to obtain the required constant-current carrying capability, another important factor in selecting the switching device is the ease with which paralleling can be accomplished. Consider a simplified circuit formed from three transistors Qa, Qb, and Qc in parallel. Assume all three transistors are operating, and sharing current, I. Now assume that Qa heats up and passes more current than Qb or Qc. With more curent, it heats more, and will soon "run away" and self-destruct. If Qa destructs in the shorted sate, the load circuit will likely be damaged. If Qa fails in the open state, then Qb and Qc will be forced to handle the current originally meant for all three. This is what is meant by "current hogging." Soon Qb or Qc will also run away, causing another device failure. This process will continue until all the devices have self-destructed or the circuit fails.
Another circuit could be added to prevent this; but the addition of such circuitry would require extra power, thus reducing the efficiency of the system. Also, bipolars are current-operated devices requiring current-drive circuitry. A good engineering rule states that the less the component count in the design, the more reliable and efficient the system. Thus, bipolar devices used to switch current to the field of an AC generator are not only inherently inefficient but also unduly complicate the design of the overall circuit. What is needed is a highly efficient, simple, and reliable voltage regulator which overcomes these shortcomings. Heretofore, this long felt need has not been satisfied by voltage regulators incorporating bipolar power switching devices.